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<div class="moz-cite-prefix">On 05/07/2019 13:29, Sven Barth via
fpc-devel wrote:<br>
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<div dir="ltr" class="gmail_attr">J. Gareth Moreton <<a
href="mailto:gareth@moreton-family.com"
moz-do-not-send="true">gareth@moreton-family.com</a>>
schrieb am Fr., 5. Juli 2019, 14:02:<br>
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<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">I'm glad
we agree on the "is" operator. Personally I would like to
<br>
extend it so it accepts "if (I is TMyEnum) then" when I is
also of type <br>
TMyEnum, since it may be the case that you're reading I
directly from a <br>
stream, either individually or as part of a record, in
which case <br>
converting I from an Integer to TMyEnum is a little
inconvenient. When <br>
you get down to the compiled assembly language it
shouldn't really <br>
matter anyway.<br>
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<div dir="auto">It might not matter from the assembly point of
view, but it matters from the language point of view. When we
say that enum values only contain valid values then "myenum is
TMyEnum" *must* always evaluate to true, because otherwise
there's an exception of the rule that enums only contain valid
values. </div>
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<div dir="auto">Regards, </div>
<div dir="auto">Sven </div>
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<p>In this situation, the exception is kind of needed, and was
suggested for this purpose in the first place. If you read an
enum from a stream or any kind of external source, then there is a
chance it will take on an invalid value and needs to be trapped.
I don't think anyone wants to be forced to ditch enumerations for
integers and C-style constants so as to not fall foul of this
quirk. "is" is to catch an enumeration taking on an invalid
value, thus allowing customised error handling to be executed, so
other expressions can be optimised on the assumption that the enum
is definitely valid by that point.<br>
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<p>Gareth aka. Kit<br>
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